1. Field of the Invention
The present invention relates to an exhaust emission control device for purifying exhaust from internal combustion engines by trapping exhaust particulates.
2. Description of the Related Art
Emission control requirements for internal combustion engines of automobiles are becoming increasingly stringent. Emission reduction of CO, HC, NOX and particulates such as soot and SOF contained in the exhaust is particularly required for compression-ignition (diesel cycle) engines that are fueled by diesel fuel. To trap particulate matter (PM) in the exhaust from diesel powered engines, a diesel particulate filter (DPF) is normally disposed in the exhaust passage. A DPF is regenerated by periodic oxidation of accumulated PM to retain its PM trapping capability. The oxidation takes place more stably in a catalyzed DPF because the catalyst carried on its filter substrate lowers the regeneration temperature. DPF is normally regenerated at a temperature of 600° C. or more by a supply of unburned HC by means of retarding injection timing or post injection.
Injection timing retard causes a loss in the engine output torque because it increases waste heat in combustion. Post injection is an injection of fuel that does not contribute to the combustion in the combustion chamber. Therefore, frequent regeneration of DPF before PM has been fully accumulated with these methods will increase fuel consumption and lower fuel economy. If regeneration is performed after a large amount of PM has been collected, the PM will be oxidized intensively and the temperature of the DPF will become too high, e.g., 1000° C., whereby the risk of damage to the DPF substrate or deterioration of the catalyst will increase. The DPF regeneration timing should therefore be suitably set based on the calculated amount of trapped PM. Since the PM amount differs from one engine to another, it must be calculated in real time.
Known techniques in the art determine the regeneration timing using the fact that PM accumulation causes an increase in pressure loss. Japanese Patent Laid-Open Publication No. Hei 6-341311 (1994) shows one example of such techniques; when the differential pressure across the DPF exceeds a preset upper limit, the DPF is regenerated. Today it is common to use a semiconductor pressure sensor for DPFs for detecting differential pressure. Semiconductor pressure sensors include a piezoelectric element that outputs electrical signals in accordance with flexure caused by pressure in a semiconductor thin film that forms a pressure detection surface. Furthermore, the sensors can be compactly designed.
The differential pressure across a DPF changes depending on the flow rate of exhaust passing through the filter, even though the amount of PM accumulated in the filter is the same. The differential pressure decreases when the exhaust flow rate is low when the vehicle is running at low speeds in urban areas, and increases when the exhaust flow rate is high when the vehicle is running at high speeds on highways.
Seasonal and regional temperature variations that can be very large must also be taken into consideration because they affect the temperature of the sensor that detects the differential pressure across the DPF. Pressure sensors used for DPFs therefore need to have a wide detection range and superior temperature characteristics, but these requirements are not fully met in reality.